Fans : How many blades?

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Ed Pawlowski wrote:

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The 75" prop on a Cessna 172 can rotate up to 2700 rpm. Usually, it's undesirable for the tip to exceed the speed of sound, so bigger props are likely to have reduction gears to rotate slower.
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E Z Peaces wrote:

Rats, I misread the Mcmaster data. They don't turn faster than a Cessna 172 but do turn faster than larger propellers.
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wrote:

Yes, with a three-bladed fan, the blades don't come by so often. This gives the air time to relax and it's more compliant when the next blade comes.
With a five-bladed fan, the air is always on the alert, it's not surprised and it resists being pushed around. Like a militant teenager being hassled by the police.
This is the same reason the guy told me when I was going to stay at his apartment overnight, that I should sleep with him in his bed, instead of on the sofa. On the sofa, i would relax and not resist him. In his bed, next to him, I would always be on the alert and he woudln't be able to do anything to me.
The bed was the five-bladed fan and the sofa the 3-bladed, according to him.

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mm wrote:

WTF???
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:) for the humor impaired - I thought it was LMAO funny...
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But true. That's what he told me. I felt like I was a girl with a guy lying to me about his intentions.
I thought we were just having dinner together at one of those very low cost buffets in Las Vegas, until he started paying me compliments. I felt like a girl then too.
Anyhow I slept on the sofa.
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My house has 4 ceiling fans. 1 has 3 blades, 2 have 4 blades and 1 has 5 blades. They all have 3 speeds and all are reversible and very quiet.
As far as I can tell they all do the same excellent job of moving air about. Without scientific instruments to take measurements, I don't think anyone would be able to tell if one was more "efficient" than the others.
Freckles
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Ideal number of blades is, of course, 42.
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On Thu 23 Jul 2009 01:56:01p, Bob-tx told us...

Two, three, four, or five blades matters far less than the CFM (cubic feet of air per minute) delivered at all speeds, usually three. The number of blades is often more an issue of aeshtetic preference than performance. Other factors such as the degrees pitch of the blades and the RPM determine how much air is delivered.
We have seven ceiling fans in our house, ranging from a top price of around $600 down to one that costs only $70. The high end fans have five blades (54" blade span) and were chosen not ony for performance but also for aesthetics and motor warranty. The least expensive fan is smaller (42" blade span), has a smaller motor, shorter warranty, and delivers somewhat less air, but it was ideal for the location it was selected.
Do you homework... You'll rarely get a good answer from store personnel unless it's a fan specialty store with well-educated staff.
--
Wayne Boatwright
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